1. Field of the Invention
The present invention relates generally to a labyrinth seal, and more specifically to a labyrinth seal is a turbomachine.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
Labyrinth seals are often used to reduce or control fluid leakage in systems having equipment such as gas and steam turbines, compressors, pumps, and other types of rotating equipment where fluid flow may occur between two relatively rotating members. More particularly, labyrinth seals are frequently used in sealing between elements such as a rotating shaft and an associated stator housing to inhibit undesirable fluid flow between the exterior of the shaft and interior of the stator. When incorporated with a pump, a labyrinth seal may be relied upon to inhibit leakage along the pump shaft of high pressure fluid being discharged by the pump.
A labyrinth seal is generally characterized by a series of cavities or grooves formed along the adjacent surfaces of two relatively rotatable members such as a rotor on a rotating shaft and a stator on an adjacent, fixed housing. These members generally define a partial barrier between areas of high and low fluid pressure. At successive stations along the length of the labyrinth seal, adjacent surfaces of the rotatable members are situated in close proximity to each other to define annular slit-like orifices. In many labyrinth seal designs, a series of cavities or chambers are formed at these stations in order to retard fluid flow through the seal to a desired level.
In operation, previously available labyrinth seal designs often form a fluid barrier between the rotatable members by forcing high velocity fluid to navigate irregularly spaced adjacent surfaces formed between the relatively rotatable members. The fluid is forced to pass sequentially through slit-like orifices to enter enlarged cavities where the fluid energy is largely dissipated in turbulence. One of the basic concepts of any labyrinth seal design is to create a highly frictional fluid flow path. Such a flow path will convert pressure energy into velocity energy. A large portion of the velocity energy will be dissipated as heat energy via turbulent action. The basic concept for this design is shown in the Prior Art FIG. 1. One problem with the straight-through labyrinth seals shown in FIG. 1 is the carry-over effect. A vortex 8 is created within the cavity of the labyrinth seal, and as the vortex increases in diameter the leakage flow though the gap will tend to not flow within the cavity, but flow straight toward the next gap. The effect of the labyrinth seal is reduced.
One source of turbulence is created as a result of wall shear friction between high velocity fluids and irregularly spaced adjacent surfaces of the seal. A second and often more important source of turbulence results from intense free shear layer friction between a high velocity fluid jet discharging from an orifice and relatively slow moving fluid in a large cavity immediately downstream from the orifice. As a result of the combination of these and other friction components, pressure energy is substantially reduce downstream of each orifice in a multi-cavity labyrinth seal system. The substantially reduced pressure in a given cavity formed downstream from a particular orifice results in smaller pressure changes occurring across additional downstream orifices. The net result is overall reduced leakage across the labyrinth seal.
In one Prior Art design, U.S. Pat. No. 1,482,031 issued to Parsons et al on Jan. 29, 1924 and entitled PACKING FOR ROTATING BODIES, a labyrinth seal is characterized by a radially stepped surface provided along the rotor, the stator being provided with a corresponding set of barrier members or collars disposed in close relationship thereto. In this fashion, high pressure fluid moving across the sealing surface will encounter interference; thus, minimizing leakage. In yet another design, U.S. Pat. No. 3,940,153 issued to Stocker on Feb. 24, 1976 and entitled LABYRINTH SEAL, the labyrinth seal is characterized by a succession of annular orifices or clearances between sealing teeth or knives on one member, and generally cylindrical surfaces or lands on the other. In combination, the sealing system defines a doubly re-curved flow path from each orifice to the orifice next downstream. This design is represented by the Prior Art FIG. 2.
Such prior art systems employ the use of sharp turns in the fluid flow path to provide additional fluid friction or resistance to flow. The through-flow fluid is forced to “zig-zag” or “serpentine” through the seal. The turning of the through-flow fluid in the prior art is often achieved through the use of wall positioning and wall curvature. Many of the prior art configurations were designed without precise quantitative data and without fully appreciating the kinetics involved in turbulence generation and energy dissipation associated with a sealing system. The concern of the prior art has generally been to increase the wall shear friction through the use of long and tortuous flow paths between each pair of annular orifices. By focusing on the use of wall shear stress, the prior art often neglected the turbulence generating potential of a free (i.e. away from wall) shear layer.
One problem associated with the stepped labyrinth seals is the carry-over effect described with respect to the FIG. 1 labyrinth seal. The vortex flow 8 can build up in the cavity of the stepped labyrinth seal. If the vortex flow is large enough, the carry-over effect will also occur within the stepped labyrinth seal. The leakage flow will be forced to flow from one gap to the next without passing into the cavity, and thus, the stepped labyrinth seal will act like the straight-through labyrinth seal of FIG. 1.
Another prior art design, U.S. Pat. No. 3,897,169 issued to Fowler in Jul. 29, 1975 and entitled LEAKAGE CONTROL STRUCTURE, which is represented in prior art FIG. 3, shows a stepped labyrinth seal with a first gap 324 formed between a first stepped portion 326 and a first fin 328 extending radial inward toward the first stepped portion 326, a second stepped portion 330 downstream from the first stepped portion 326 which includes a concave side surface 331, where the second stepped portion 330 is stepped or raised radially outward with respect to the outer surface stepped portion 326. a second fin 3332 forms a second gap 336 with the second stepped portion 330.
The difference between the present invention and the stepped labyrinth seals such as the Fowler design shown in the prior art FIG. 3 is that the side surface 331 has a concave surface in order to facilitate machining. However, side surface 331 could be flat or have other appropriate machined contours and function properly. The purpose of the side surface 331 in Fowler is for the kinetic energy of the fluid flow leakage through the gap 324 to be dissipated by the reaction of such leakage against the side surface 331. in operation, with the kinetic energy of the fluid leakage through gap 324 substantially dissipated by the side surface 331 and with the inclusion of the second fin 332, a back pressure sealing effect within the annular cavity 334 is created with respect to gap 324. as Fowler discloses above, the leakage flow passing in the first gap 324 is captured in the side surface 331 and brought to a rest (kinetic energy=zero), converting the dynamic head to a static head rise, representing a larger delta P. the pressure of the fluid downstream from the gap 324 is therefore high, resulting in more leakage flow through the downstream second gap 336.
Applicant's invention does not convert the kinetic energy into static energy as the leakage passes through the gap. The kinetic energy in the leakage flow is actually used to turn the flow about 180 degrees and guide the flow into the next cavity of the labyrinth seal assembly without slowing the flow.